165639 - NSSRWLock_UnlockWrite can cause deadlock

Status: RESOLVED FIXED

(NSS :: Libraries, defect, P2)

Description

[Julien Pierre]

When using the re-entrant feature of this lock, deadlocks can occur.
I ran into it with the CRL cache.

The test case is :

acquire the lock for write
acquire the lock for read
unlock the lock for write
unlock the lock for read

Wan-Teh produced a patch for this problem that I will attach.

Comment 1

[Julien Pierre]

Attachment: resolve deadlock

Comment 2

[Wan-Teh Chang]

Attachment: resolve deadlock v2

This patch is equivalent to and shorter than the previous patch

Comment 3

[Wan-Teh Chang]

The problem was that NSSRWLock_UnlockWrite failed to
wake up waiting readers when the thread releasing
the writer lock still owned the reader lock.  Under
this condition, it is fine to let other threads
share the reader lock.

Since this implementation gives preference to waiting
writers, my patch only wakes up waiting readers if
there is no waiting writer.

I have checked the new patch into the tip.  Julien,
please verify the fix.  I'd also appreciate a code
review by the author of nssrwlk.{h,c}.

Comment 4

[Nelson Bolyard (seldom reads bugmail)]

r=nelsonb